Why Copyleft?

“When it comes to defending the freedom of others, to lie
down and do nothing is an act of weakness, not humility.”

In the GNU Project we usually recommend people
use copyleft licenses like GNU
GPL, rather than permissive non-copyleft free software licenses. We
don't argue harshly against the non-copyleft licenses—in fact,
we occasionally recommend them in special circumstances—but the
advocates of those licenses show a pattern of arguing harshly against
the GPL.

In one such argument, a person stated that his use of one of the BSD
licenses was an “act of humility”: “I ask nothing of
those who use my code, except to credit me.” It is rather a
stretch to describe a legal demand for credit as
“humility”, but there is a deeper point to be considered
here.

Humility is abnegating your own self interest, but you and the one who
uses your code are not the only ones affected by your choice of which
free software license to use for your code. Someone who uses your
code in a nonfree program is trying to deny freedom to others, and if
you let him do it, you're failing to defend their freedom. When it
comes to defending the freedom of others, to lie down and do nothing
is an act of weakness, not humility.

Releasing your code under one of the BSD licenses, or some other
permissive non-copyleft license, is not doing wrong; the program is
still free software, and still a contribution to our community. But
it is weak, and in most cases it is not the best way to promote users'
freedom to share and change software.